Recently the use patterns, circumstances and consequences of cocaine have become of interest to both researchers and policy makers in Europe. Currently only a few studies have been conducted of the social epidemiology of cocaine. In this article, focus is placed upon the use of cocaine in one subpopulation, heroin addicts. Based mainly upon ethnographic research conducted in the city of Rotterdam it can be estimated that the prevalence of cocaine use in this population has reached a very high level. The mode of ingesting cocaine parallels that of heroin; Injecting Drug Users inject cocaine-hydrochloride, heroin smokers smoke cocaine base. This cocaine base is mainly processed by users themselves. An exception to this rule of self-processing can be found in the most marginalized addicts who do not have access to the house addresses where both heroin and cocaine-hydrochloride are sold. In this group the selling of 'cooked cocaine', a crack-like product, has occurred. The circumstances and potential consequences of the emergence of 'cooked cocaine' are discussed.